1. Field of the Invention
The present invention relates to a gas concentration sensor, to a motor vehicle having a gas concentration sensor, and to a method for measuring gas concentration.
2. Description of the Prior Art
Tightening of the emission control regulations in the European Union and in the USA has led to requirements that emission limiting values are not exceeded during a cold start of an internal combustion engine. In these cold start phases, a filter for filtering fuel vapors is frequently scavenged with fresh air. For example, the filter is an activated carbon filter by which fuel vapors are prevented from escaping from a tank system into the environment. The air provided with a proportion of fuel is fed into an intake section of the internal combustion engine from the activated carbon filter. A proportion of fuel is therefore introduced into a fuel/air mixture of the internal combustion engine by the air from the activated carbon filter.
However, during the cold start phase of the internal combustion engine, a lambda sensor arranged in an exhaust system of the internal combustion engine is usually not yet in an operationally ready state. For this reason, the exhaust gas emissions cannot be reduced to a minimum by a control device of the motor vehicle owing to measured values detected with the lambda sensor. For this reason it is appropriate to determine a proportion of fuel in the air leaving the activated carbon filter.
To determine a composition of a gas mixture, acoustic measuring methods are known that make use of the fact that the speed of sound in gas mixtures depends on the composition of the gas mixture. This effect may be used to measure the mixture ratio of two gases whose substances are known. For example, the speed of sound in 100% air at a temperature 0° C. is 331.6 m/s. This exemplary speed of sound is reduced to approximately 225 m/s when propane is mixed and if only propane is then present. The known acoustic measuring methods usually operate in the ultrasonic range. Furthermore, the speed of sound in a gas or gas mixture is influenced by temperature and pressure of the gas or gas mixture. However, the pressure and temperature are computationally related so that given knowledge of the pressure or temperature the respective other variable can be determined.
A known acoustic measuring method operates in a way analogous to an echo sounder. In this context, a wave packet of defined frequency and length is transmitted into a measuring chamber from a sound generator to a receiver at a defined distance. The receiver is usually arranged on a side of the measuring chamber lying opposite the sound generator, at a same height as the sound generator. A transit time between the emission of the wave packet and the arrival at the receiver is measured. The mixture ratio of the gases present in the measuring chamber is determined from this transit time.
Frequencies in a range from 100 to 400 kHz are used for the “echo sounding” method. In this frequency range, a narrow frequency lobe is formed at the transmitter and, therefore only a small number of scattered signals are formed. Furthermore, the scattered signals travel larger distances than the measurement signal, as a result of which the influence of the scattered signals on the measurement signal is reduced because amplitude attenuation is higher the higher the frequency used.
A disadvantage of the above “echo sounding” method is that the transducers used are very expensive compared to simple piezo-flexural vibrators or electrodynamic transducers, and that the technical complexity of the measurement is correspondingly greater.
A further known acoustic measuring method is a phase measuring method. This likewise operates with a measuring chamber in which a sound generator and a receiver are located at a defined distance. A change in the speed of sound brought about by the composition of the gas mixture passed through is determined from a phase shift between a transmission signal of the sound generator and a reception signal of the receiver. Owing to the change in the speed of sound, a mixture ratio of the gases in the measuring chamber can be determined.
A disadvantage of this method is that the measuring range is restricted to a maximum phase shift of one wavelength. If, for example, a phase shift of 380° were to occur, this would not be detected by the phase measuring method. In this example, a phase shift of 20° would be detected with the phase measuring method.
Therefore, this phase measuring method also requires large wavelengths, that is to say low frequencies, to detect large changes in transit time. However, the lower the frequency being used, the greater the effects of scattered signals. These scattered signals are reflected at the measuring chamber walls and interfere with the measurement signal along the measured section and leads to further phase shifts. Furthermore a signal amplitude of the measuring signal is reduced to unmeasurable values. For this reason, either a voluminous measuring chamber or special attenuating elements are used to minimize the influence of scattered signals.
An acoustic measuring method which is also known is based on a change in a resonant frequency of a quartz oscillator on the basis of an accumulation of gas molecules. The accumulation of gas molecules at the quartz oscillator brings about an increase in the mass of the quartz oscillator, which in turn results in a change in the resonant frequency of the quartz oscillator.
Other approaches to determine a gas composition operate with optical methods. These optical methods are, however, unsuitable in the field of motor vehicles owing to their susceptibility to soiling. Alternatively, methods are known which operate using heated ceramics, which for safety reasons, are not able to be used in the field of the fuel supply in a motor vehicle in particular in view of the risk of explosion.